An experimental study on the effect of nitrogen injection on the deformation of coal during methane desorption

Abstract

Gas-injection to enhance coalbed methane recovery (ECBM) is a hot issue at present. Several studies have pointed out that nitrogen injection can improve the permeability of coal reservoirs. However, there is still a lack of research on coal deformation induced by nitrogen injection during methane desorption. In this study, five coal samples from Shanxi Province, China were collected to conduct a comparative study on the deformation of coal during methane desorption with/without nitrogen injection, and the influence of nitrogen injection on the deformation of coal was analyzed. Through this study, the reason for the increase the permeability of coal caused by nitrogen injection was analyzed in combination with the deformation characteristics of the coal. The results show that the strain-time curves during adsorption are similar to the Langmuir curves. The swelling of coal during adsorption decreases first and then increases with the increase of random reflectivity of vitrinite (Ro, ran). The shrinkage is less than swelling for the same sample after the adsorption-desorption cycle without nitrogen injection. There is a certain residual deformation since part of methane is unable to become a free state after being adsorbed by coal. The residual strain of the samples is between 39 and 402 uε in the two desorption processes. After nitrogen injection, in the process of desorption with successively decreasing outlet pressure (simulated the change of bottom hole pressure in the field CBM production) and with constant outlet pressure (atmospheric pressure), the accumulated shrinkage of the sample is higher than the corresponding swelling during adsorption. On the one hand, nitrogen injection can reduce partial pressure and promote the desorption of methane. On the other hand, nitrogen injection can induce the shrinkage of coal, which may be one of the reasons that lead to the increase of permeability. The strain induced by nitrogen injection is between 199 and 853 uε, which has a weak negative relationship with the ash yield. The strain induced by nitrogen injection decreases first, then increases and then decreases again with the increase of Ro, ran.